I have one and I like to record my guitar by having it pointed straight into the guitar amp. Am I getting the optimum quality/sound range by using the ATR30, or should I get a different mic for this? And if so which one? Thanks in advance.

I'd go with a Shure SM57 if you're on a budget and want a pretty versatile mic. Stick it an inch or two away from the speaker cone, about halfway between the centre and the edge and pointing towards the centre just a fraction. As long as your amp's EQ'd well, you should get a pretty nice sound from that